I am a noob to ponds and most of all fish so please take that into account when rolling your eyes at the many errors I may have made to this point.

First and foremost I live in a coastal spot in New Zealand where the climate is pretty timid all year around. I had a little spare time on my hands this summer so I decided to create a water feature. I did a little investigating and chatted with a few people about good and bad things to take into account. About 6 weeks later and I have a 7-8000 litre Firestone lined pond, running a "Pondone" Clari Tec 10000UV filter with a 24/7 waterfall supplied by a 6000l/hr pump (I'm actually only getting 3600 due to head pressure resistance).

I used concrete to lock in the rocks surrounding the pond and to fabricate parts of the waterfall so I washed everything down well and pumped out before filling the pond. I then ran the pump for a week before introducing 5 Lilys and one Lotus. A week or so later I added a 3ft fish tunnel & 4 (50mm) Comet goldfish. They seemed happy enough after 4 days so I introduced 6 Fantails (120-160mm) and a few buckets of fresh water weed from a local river.

I have since had 3 of of the comets and one of the fantails die over the span of a week. I purchased a testing kit and found the only thing out of spec was pH which measured 9 (or more) on the high pH scale. Everything else was 0. At this stage I also realised that I had the Lotus sitting on a cinder block. I have been swapping out between 100 to 500 liters at a time, every other day over the last week as advised by a local store owner that I got my gear through. The fantail died this morning so I swapped out almost half the water and the pH came back as 8.6. Tap water here is 7.2.

Am I doing the right thing or should I pull the whole lot, buy a rescue tank and start again? This has also coincided with my first algae bloom and I have now turned on the UV lamp on the filter.

I hate seeing any animal die early that would otherwise last many year in happy bliss so any help would be greatly appreciated.
M.

I hope so. I ended up replacing the cinder block with an old retired 10l paint bucket. Despite the fact that the bucket has been used as a gardening bucket for the last 5 years or so (cleaned a million times), I made sure to clean it thoroughly, remove the galvanised handle and cut four opposing holes for fish to hide. I also drilled 30 or so holes in the top for any gasses to escape.

pH Update:

I tested the pH both late yesterday afternoon and first thing this morning with the results being 8.4 and 7.8 respectively. I have read the pH can drop overnight only to climb again throughout the day so i guess I will need to keep swapping out for a few more days yet.

There are many possibilities here regarding the ultimate cause of the problem, but the main suspects are:

1-- disease. The fish, or the plants, could have brought disease into the pond.

2-- ammonia. This is a newly setup system, and such systems are usually plagued by this. Normally it's not too huge a problem, BUT with a pH of 9, even the slightest trace of it can be hazardous. ( it's more toxic at higher pH )

3-- that pH of 9 was certainly high enough to harm the fish all by itself, and then there are the leachates from the cinderblock. THIS is a major contender for being the main cause of the problem.

You are already doing all the right things. Sometimes all you can do is fix the causes and wait out the storm.